Latitudinal gradients in species diversity are generally understood to be increases in the number of species from high (cold-temperate) to low (warm) latitudes. Increases in diversity at low latitudes are also known for genera and higher taxa, therefore “latitudinal gradients in biodiversity” may be a better term.
Alexander von Humboldt discovered the latitudinal gradients in species diversity during his expedition to South America in 1799, and he even suggested that temperature may give an explanation .
The importance of studying latitudinal gradients
A central, if not the central, question in evolutionary ecology is that for the reasons of differences in the abundance and diversity of organisms in different habitats and regions. Such differences are common between habitats, even neighbouring ones, and on a larger scale they are apparent not only between latitudes, but also between different altitudes, different depths (particularly in oceans) and different longitudes. But by far the best documented gradients are latitudinal ones. This presents us with the opportunity to attempt an explanation of the causes that determine diversity.
Latitudinal gradients in species diversity are known from most groups of plants and animals, although there are exceptions. We discuss only a few examples. Krebs (1985 ) gives the number of ant species at various localities, of which we select four: Alaska 7 species, Utah 63, Cuba 101, and Brazil 222 species. Also according to Krebs, there are 22 species of snakes in Canada, 126 in the U.S., and 293 in Mexico, and Malaysian tropical rainforests may contain 227 of tree species on two hectares, whereas a deciduous forest in Michigan USA will contain only 10-15 species in the same area. Wright (2002 , references therein) gives much greater numbers for tropical rainforests: a 0.52km2plot in Borneo had 1175 species (with a diameter at breast height of at least 1 cm), and one hectare of Amazonian rainforests can have more than 280 species of trees in one hectare (with a diameter at breast height of at least 10 cm). Examples for parasites are helminths (parasitic worms) of marine fishes. Whereas on average one species of Digenea is found on one fish species at all latitudes in the Atlantic and Indo-Pacific Oceans, the number of species of gill Monogenea per fish species ranges from 1 at high latitudes to over 2 at low latitudes in the Indo-Pacific, and from 0.3 at high latitudes to 1.5 at low latitudes in the Atlantic (Rohde 2005 ). In other words, the gradient for Digenea is entirely due to the increased diversity of hosts in the tropics, whereas that for Monogenea is partly due to increased fish diversity, and partly to a relatively greater diversity (species number per host species) in the tropics (Figure 1). – In the oceans, latitudinal gradients are well documented for surface waters, but there is some evidence that they also occur in the deep-sea (e.g., Lambshead et al. 2002 for deep-sea nematodes ). Two examples of surface gradients in the oceans are illustrated in the following.
Figure 1. Latitudinal gradients in species richness of marine coastal fishes (teleosts and elasmobranchs). Crosses and interrupted lines: Atlantic. Dots and thick lines: Indo-Pacific. Based on but strongly modified from figure 4 in Rohde (1978 ) and figure 72 in Rohde (1993 ), data from various authors particularly in Briggs (1974 ). Only surveys from large areas are included. The highest diversity in the Indo-Pacific shown here is for the Philippines, for the Atlantic it is for the Western Caribbean, the lowest in the northern Atlantic and Indo-Pacific, respectively, are those for Greenland and the Sea of Okhotsk. Note the much greater diversity in the Indo-Pacific than the Atlantic, and an increase of diversity towards lower latitudes. – Also note that data for high latitudes are more complete than those for low latitudes, i.e., it can be expected that the gradients are even steeper than shown here. © Klaus Rohde
Figure 2. Latitudinal gradient in species diversity of molluscs on North- and South American Pacific shelves. Points are selected in bands of 5olatitude. The bar indicates the latitudinal range of the highest mean sea surface temperatures (SST, 27-28oC). Note that they do not occur at the equator but between 20oNorth and close to the equator. Correspondingly, highest diversity is shifted to the North. Data for North America from Roy et al. (1998 ), for South America from Valvodinos et al. (2003 ). Original Klaus Rohde, based on Valdovinos et al. (2003 ) but strongly modified with the permission of the authors. © Klaus Rohde
Among exceptions are parasitoid hymenopterans on land and helminths (parasitic worms) of marine mammmals, i.e., cetaceans and pinnipeds (Figure 3), which reach peak diversity outside the tropics. Also, importantly, many groups are comprised of cold- and warm-adapted subgroups, the diversity of the former of course peaking at high latitudes. One can expect “typical” gradients therefore only if sufficiently large groups are considered.
Figure 3. Helminth species of pinnipeds and cetaceans at different latitudes. Data from Dogiel (1964 ). Note: boreal = northern cold-temperate region, antiboreal = southern cold-temperate region. © Klaus Rohde
Age of gradients
Latitudinal diversity gradients have been in existence for long geologic periods, although their steepness underwent changes (e.g., Powell 2007  for brachiopods in the late Paleozoic ice age; and Brayard et al. 2006  for Triassic ammonoids).
Explanations of the gradients
No agreement has been reached about an explanation. The following link leads to the responses of scientists involved in research on the problem to the question “What causes spatial gradients in species diversity?” in the popular science magazine Seed.: http://www4.ncsu.edu/~rrdunn/What%20causes%20latitudinal%20gradients%20in%20diversity.html. Various authors have reviewed the evidence given for the large number of hypotheses proposed to explain the gradients (Pianka 1966 , Rohde 1992 , 1999 , in press, Gaston 2000 , Willig et al. 2003 , and Mittelbach et al. 2007 ). Gaston (2000 ), discussing not just latitudinal gradients but global patterns in biodiversity in general, writes that “no single mechanism adequately explains all examples of a given pattern”, “that the patterns…. may vary with spatial scale, that processes operating at regional scales influence patterns at local ones, and that the relative balance of causal mechanisms means that there will invariably be variations in and exceptions to any given pattern”. Rohde (1992 ) listed and discussed 28 hypotheseson particular mechanisms thought to be responsible for the gradients. He considered 12 of these to be circular (e.g., predation, competition), 11 based on insufficient evidence (e.g., area, productivity), and five are time hypotheses, assuming that longer evolutionary time spans available for diversification will lead to greater diversity. – Among circular hypotheses are those which give greater predation and/or competition in the tropics as an “explanation”. It is likely that a greater number of species will include a greater number of predators and competitors, but this is a consequence and not an “explanation” of greater diversity, although conceivably increased predation and competition, once established, may accelerate the process of diversification. – A correlation between productivity and greater diversity does not always exist, and – importantly – it may be a consequence of greater diversity and not its cause. This is for example indicated by the fact that highly productive and diverse coral reefs are surrounded by low-productivity parts of the ocean. Apparently, corals developed in such seas and, by means of symbiotic algae (zooxanthella), built up reefs and increased productivity. – With regard toarea, the area-species relationship, i.e. a correlation of area and species numbers, is well known, but it cannot explain latitudinal gradients in species diversity (suggested for example by Rosenzweig ), because – among continents – only Africa has a larger area in the tropics, and in the seas, highest diversity is found in SE Asia, which is much smaller than other parts of the Indo-Pacific . Nevertheless, area may well explain some of the diversity in some regions. – Any explanation of the gradients must incorporate a time factor, because time is necessary to build up species numbers. However, there is no convincing evidence that the tropics have had a longer undisturbed history than other regions. A time element is an essential component of the hypothesis of effective evolutionary time (see below). Not included in the hypotheses discussed by Rohde is the mid-domain model, which assumes that species closer to the geographic midpoint of distribution can extend further away from the midpoint than species having their midpoint closer to some climatic or geographic boundary, leading to denser species packing around the midpoint. The hypothesis can be used as a null model, generating gradients in a completely stochastic manner. An ideal region to test this hypothesis is Madagaskar with its clearly defined boundaries and a large endemic fauna. But a study by Kerr et al. (2006 ) did not find support for it, although Lees and Colwell (2007) refuted their claim . With regard to latitudinal gradients, it is clear that both terrestrial and marine regions of SE Asia, centres of very high biodiversity, are not located in the centre of a continent or an ocean. The mid-domain model can therefore not give an explanation. Mittelbach et al. (2007 ) distinguish two major hypotheses, i.e., the “time and area hypothesis”, and the “diversification rate hypothesis”. According to the former, tropical climates are older and larger, thereby permitting a higher degree of diversification. According to the latter, in the tropics diversification is faster for a number of reasons: faster speciation due to more opportunities for reproductive isolation, faster molecular evolution, and the increased importance of biotic interactions. Among the many mechanisms proposed to explain latitudinal variation on diversification rates are genetic drift, climate change, speciation types, area, physiological tolerances and dispersal limitation, evolutionary speed, biotic interactions.Rohde 1992 , 1999 , in press, following some earlier authors, pointed out that gradients that are common to many different groups and occur in many different environments, must have some underlying cause or causes resulting from environmental gradients such as temperature or energy availability. This does not exclude the possibility that additional factor may not modulate the gradients caused by such primary factor(s). – Any hypothesis explaining the gradients must explain the following findings: 1) there is a gradient of increasing diversity from high to low latitudes; 2) species richness is not just somewhat but many times greater in the tropics; 3) niches of tropical species are not narrower than those of cold-temperate species  (see knol: http://knol.google.com/k/klaus-rohde/latitude-niche-width-hypothesis/xk923bc3gp4/48#; even if future studies should demonstrate somewhat narrower niches in the tropics, it is not likely that they are many times narrower). These findings are accommodated in the hypothesis of effective evolutionary time(, see knol: http://knol.google.com/k/klaus-rohde/effective-evolutionary-time/xk923bc3gp4/11#). This hypothesis incorporates elements of both the major hypotheses distinguished by Mittelbach et al. (above). In particular, it does not imply that niches must shrink in order to allow more species into a region, species can be absorbed into the largely empty niche space without affecting niche width (see knol: http://knol.google.com/k/klaus-rohde/vacant-niches-in-ecology/xk923bc3gp4/8#). The underlying environmental gradient is one of temperature: higher temperatures lead to faster mutation rates, faster selection and shorter generation times, all of which accelerate speciation; a long undisturbed history permits accumulation of species over time. Much evidence is available for the hypothesis. In particular, the metabolic theory of ecology (Brown et al. 2004 ) is in agreement with it.
Explanations for differences in gradients and exceptions
Agreement on explanations for exceptions to the gradients has not been reached in most cases, but are likely to be found in the history and characteristics of the groups. For example, two (mutually not necessarily exclusive) explanations have been proposed for the greater diversity of helminths of marine mammals in northern cold-temperate than tropical seas (Figure 3), i.e., 1) a reduced density of host populations in the tropics leading to less favourable conditions for the development of rich parasite faunas, and 2) that hosts and their parasites have originated in the northern regions and had more time to acquire parasites there. – The difference in the gradients of marine fishes between the Atlantic and Indo-Pacific (Figure 1) has been explained by the older age of the Indo-Pacific, or/and its larger area .
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Other zoogeographical knols
I wish to thank Dr. Navarete for the permission to use and modify Figure 2.